I don't know much about this first-hand, but from what I've heard
Leksah isn't usually kept much up to date with it's dependencies.
Probably your problem comes from this dependency [1]:
ghc (≥6.10.1 & <7.5)
Note that in [extra] you find ghc 7.6.1.
This is just a guess, though.
Alessandro
[1] http://hackage.haskell.org/package/leksah